Switches are used in a wide variety of applications. In the past, powered switches were used in most applications. In general, powered switches are directly connected between a load and a power source or between the load and ground. For example, a powered switch can be connected between a power source and a motor, such that a relatively large load current flows through the switch and motor and when the switch is closed. Thus, in powered switches the load current flows through the switch when the switch is closed.
Recently, sensed switches have gained wider use in the marketplace. In contrast with powered switches, a large load current does not flow through the sensed switch. Instead, sensed switches are switches where the state of the switch is read by a detection circuit. The determined state of the switch is then used by the system in some way that fulfills the purpose of the switch.
An ideal sensed switch has infinite resistance when open and zero resistance when closed. Unfortunately, sensed switches degrade with time due to temperature cycling, humidity and contamination. Such degradation can cause unwanted open-state current leakage and unwanted closed-state switch resistance. Open-state current leakage can cause an open switch to be falsely read as closed. Likewise, closed-state resistance can cause a closed switch to be falsely read as open. In either case the degradation can cause errors in reading the switch state and negatively impact the performance of the system. For these and other reasons, it is desirable to provide techniques to determine the health status of sensed switches before degradation in the sensed switch can cause such errors.